Investigation On Radiation Characteristics And Optimization Of High-order Harmonic Generation In Metal Plasma

Many complex physical processes are involved in the interaction between laser and matter,such as non-sequential double ionization,sequential double ionization,above-threshold ionization and high-order harmonic generation（HHG）.As an effective method to generate ultra-short extreme ultraviolet pulses,HHG has become a useful tool for studying ultrafast dynamics and an important research topic in strong field atomic physics.Although the theory and method of strong-field atomic physics have been extended to more complex substances,such as molecules,clusters,nanoparticles,and even solids and liquids in recent years,the research on the interaction between a single atom and a strong laser field is still of great significance to the development of the basic principles and applications of strong field physics.Due to the low densities of the plasmas,the fundamental HHG process is considered to be originated from the independent response of the individual atoms/ions in the plasma,similar to HHG in gaseous atoms.The method of producing low-density plasma by laser ablation of solid materials can be applied to almost any solid material,which greatly broadens the types of atoms/ions studied by HHG.Second,the harmonic conversion efficiency of certain metal ions is very high,which provides broad prospects for practical applications.This paper is based on the high harmonic characteristics and optimization of metal plasma,which is mainly divided into the following aspects:1.Being the primary characteristic of HHG,the cut-off energy of the harmonic spectrum has been received extensive research.In contrast to gaseous HHG,it is a challenge to track which plasma particles are responsible for the harmonic cutoff energy in a plasma HHG experiment.We have investigated and compared the cut-off energies of high-harmonic generation in the plasma of various metals driven by the near-infrared femtosecond laser field.The laser intensities during the ionization saturation of different ions are estimated using the ionization calculations based on Perelomov-Popov-Terent’ev theory.The measured cutoffs from most metals are in reasonable consistent with the predictions of the semi-classical cutoff law when the calculated values are taken as the effective driving laser intensities.We have successfully identified the contribution of different ions in HHG process,and proved that the selection of ion state is crucial to the understanding of the harmonic cut-off in plasma HHG experiments.2.How to improve the yield and cutoff energy of harmonics has always been a hot research topic in HHG.In the gaseous HHG system,the use of long-wavelength laser to drive high-density gas seems to be an optimal choice.However,in plasma HHG,it is difficult for long-wavelength laser to extend the cut-off region of harmonics because ions cannot effectively compensate for the phase mismatch caused by electron dispersion.Here,we propose a scheme that can simultaneously increase the harmonic yield and cut-off energy.By reducing the central wavelength of the driving laser from800 nm to 400 nm in experiment,the harmonic cutoff energy of Tin plasma has been successfully increased from～51 e V to～84 e V,and the harmonic yield is greatly improved.Through theoretical calculations,the contribution of the Sn3+ion to harmonic generation accounts for the cutoff extension at 400 nm.With the qualitative analysis of the phase mismatching effect,we reveal the phase mismatching caused by the dispersion of free electrons is greatly suppressed in the 400 nm laser field relative to the 800 nm laser field.3.In the HHG experiment,the harmonic yield depends on the single-atomic response of the target as well as the macroscopic properties of the medium which affect phase matching and absorption effects.However,it is difficult to precisely control and detect the macroscopic state of the metal plasma in the experiment,which further hinders the understanding of the generation and propagation of harmonics.We design the medium length dependence experiment of HHG to solve this problem.The relationship between the harmonic yield and the medium length L_med is measured.By separating the microscopic effect of harmonics from the macroscopic propagation effect,the roles of phase matching and single-atom term in resonance harmonics are successfully distinguished and the microscopic effect is proved to be the main reason for the enhancement of harmonic resonance.Then,we extend the experiment to ordinary metal plasma.By comparing single-atom term of different metal ions,it is found that the single-atom term of metal ions with only one electron in the outermost shell is weak.Finally,we measure the spatial distribution of Ti plasma density,and realize the phase matching of Ti plasma by using the spatial distribution of density,which provides a new idea for the phase matching of plasma HHG.